The present invention relates to magnetic bearings for levitating or suspending a rotatable component. More specifically, the invention relates to a magnetic bearing that provides radial positioning of a rotatable component on a passive basis to facilitate rotation of the component about a predetermined axis.
Magnetic bearings are commonly used to levitate or suspend rotatable components, e.g., flywheels, and thereby facilitate rotation of the component about a predetermined axis. Magnetic bearings provide substantial advantages in relation to mechanical bearings. For example, magnetic bearings facilitate substantially friction-free operation, and thus function without most of parasitic energy losses that occur in virtually all mechanical bearings.
Magnetic bearings are classified as xe2x80x9cactivexe2x80x9d or xe2x80x9cpassive.xe2x80x9d Active magnetic bearings usually comprise one or more electromagnets that create return forces. A typical active magnetic bearing also comprises one or more position sensors that operate in conjunction with a servo control system. The servo control system varies the current passing through the electromagnets in a manner that causes the return forces to suspend and align the rotatable member along a desired axis of rotation.
Passive magnetic bearings typically comprise one or more permanent magnets fixed to the rotating or static components of the bearing. The permanent magnets produce attractive or repulsive forces that bias the rotating component toward or along a desired axis of rotation. Passive magnetic bearings, in general, are lighter, smaller, less complex, less expensive, and more reliable than active bearings of similar capability. A passive magnetic bearing, however, cannot provide stable positioning of the rotatable member in the radial and axial directions, i.e., with respect a set of orthogonal axes one of which extends along the desired axis of rotation. Passive magnetic bearings, therefore, are typically used in conjunction with one or more active bearings.
So-called xe2x80x9ccentering bearingsxe2x80x9d represent a particular type of passive magnetic bearing. Centering bearings exert a radial force on a rotatable member that biases the rotatable member toward a desired axis of rotation. One possible embodiment of a conventional centering bearing 100 is depicted in cross-section in FIGS. 5 and 6.
The bearing 100 comprises a first stator disk 102 and a second stator disk 104. The bearing 100 further comprises a rotor disk 106. The rotor disk 106 is fixedly coupled to a shaft 109 that supports a rotatable component such as a flywheel.
The stator disks 102, 104 and the rotor disk 106 are each formed from a soft ferromagnetic material. The stator disk 102 includes a major surface 102a having a plurality of concentric raised portions, or teeth 102b, formed thereon. The teeth 102b each form a continuous ring, i.e., the teeth 102b each extend through a continuous arc of 360 degrees. The stator disk 104 likewise includes a major surface 104a having a plurality of concentric teeth 104b formed thereon.
The rotor disk 106 has a first surface 106a and a second surface 106b. The first surface 106a has a plurality of concentric teeth 106c formed thereon. The second surface 106b likewise has a plurality of concentric teeth 106d formed thereon. The geometry, i.e., the size and shape, of each tooth 106c substantially matches that of a corresponding tooth 102b on the stator disk 102. The geometry of each tooth 106d substantially matches that of a corresponding tooth 104b on the stator disk 104.
The rotor disk 106 is positioned between the stator disks 102, 104, as shown in FIG. 5. More particularly, the rotor disk 106 is positioned so that the first surface 106a faces the surface 102a of the stator disk 102 across an axial gap 114. The second surface 106b likewise faces the surface 104a of the stator disk 104 across an axial gap 116.
The bearing 100 further comprises a ring-shaped permanent magnet 110 having a north pole 110a and a south pole 110b. The magnet 110 is fixed to a non-magnetizable mounting surface 108. In addition, the magnet 110 is fixedly coupled to the stator disks 102, 104 so that the north pole 110a is positioned proximate the stator disk 104, and the south pole 110b is positioned proximate the stator disk 102.
The noted arrangement of the magnet 110, stator disks 102, 104, and rotor disk 106 produces a magnetic-flux circuit within the bearing 100. The primary direction of flow of the magnetic flux is denoted by arrows 112 included in FIG. 5 (the arrows 112 are not depicted in the lower portion of FIG. 5, for clarity). The magnetic flux flows from the north pole 110a into the stator disk 104. The magnetic flux travels through the stator disk 104, and is at least partially focused in the teeth 104b. The magnetic flux flows from the teeth 104b, across the gap 116, and into to the teeth 106d. 
The magnetic flux flows through the rotor disk 106, and is at least partially focused in the teeth 106c. The magnetic flux flows from the teeth 106c, across the gap 114, and into the teeth 102b on the stator disk 102. The magnetic flux subsequently flows through the stator disk 102 and into south pole 110b of the magnet 110, thereby completing the magnetic circuit.
The noted flow of magnetic flux through the magnetic bearing 100, in conjunction with the geometry and arrangement of the stator disks 102, 104 and the rotor disk 106, produces a centering effect on the shaft 109. More particularly, the magnetic flux causes the teeth 102b on the first stator disk 102 to substantially align with the teeth 106c on the rotor disk 106. The magnetic flux likewise causes the teeth 104b on the second stator disk 104 to substantially align with the teeth 106d on the rotor disk 106. This phenomenon is based on the principle that the magnetic flux seeks a path of minimum reluctance.
Minimum reluctance in the flux circuit is achieved when the gaps 114, 116 are minimized, i.e., when the distances that the flux must travel to reach the first stator disk 102 from the surface 106a of the rotor, or to reach the rotor 106 from the surface 104a of the stator disk 104, are minimized. Minimization of the gap 114 occurs when the teeth 102b are substantially aligned with the teeth 106c. Minimization of the gap 116 likewise occurs when the teeth 104b are substantially aligned with the teeth 106d (as shown in FIG. 5).
Hence, the magnetic flux flowing through the bearing 100, in attempting to define a flow path of minimal reluctance, produces a magnetomotive force that urges each of the teeth 106c, 106d into substantial alignment with a corresponding tooth 102b, 104b. Aligning the teeth 102b, 104b, 106c, 106d suspends the shaft 109 and substantially aligns the shaft 109 with a predetermined axis extending in the xe2x80x9czxe2x80x9d direction, thereby permitting the shaft 109 to rotate about that axis (the noted axis is denoted xe2x80x9cC1,xe2x80x9d and the direction of rotation is indicated by the arrow 126 in FIG. 5). The resistance of the shaft 109 to radial displacement away from the predetermined axis is commonly referred to as the xe2x80x9cstiffnessxe2x80x9d of the bearing 100, and is proportionate to the above-noted magnetomotive produced by the flow of magnetic flux through the teeth 102b, 104b, 106c, 106d. 
The magnetic-flux circuit in the bearing 100 is subject to various losses. In other words, only a portion of the magnetic flux available from the permanent magnet 110 is available to suspend and align the shaft 109. The teeth 102b, 104b, 106c, 106d represent one source of flux loss. In particular, a portion of the magnetic flux that enters each tooth 102b, 104b, 106c, 106d escapes into the space between adjacent teeth 102b, 104b, 106c, 106d. 
For example, FIG. 6 is a magnified view depicting a plurality of the teeth 102b, 106c. Adjacent one of the teeth 102b define valleys 118 located between the adjacent teeth 102b. Adjacent teeth 106c likewise form valleys 118 located between the adjacent teeth 102b. A portion of the magnetic flux passing through the teeth 102b, 106c escapes from the teeth 102b, 106c and into the neighboring valleys 118. This flux leakage is denoted by the arrows 120 included in FIG. 6. The magnetic flux that leaks or escapes from each of the teeth 106c in this manner does flow directly to a corresponding tooth 102b on the stator disk 102. Hence, this flux does not contribute substantially to the suspension and centering of the shaft 109. The capacity of the permanent magnet 110 must therefore be greater than otherwise required to account for the noted flux leakage.
Increasing the capacity of a permanent magnet in a magnetic bearing typically results in a corresponding increase in the size, weight, and expense of the permanent magnet (and the magnetic bearing). Hence, minimizing the flux leakage from the magnetic circuit of a bearing can lead to substantial reductions in the size, weight, and cost of the bearing. An ongoing need therefore exists for a passive radial magnetic bearing having features that minimize the leakage of magnetic flux therefrom.
A presently-preferred embodiment of radial magnetic bearing comprises a rotor disk having a first plurality of concentric teeth extending from a surface thereof, and a stator disk having a second plurality of concentric teeth extending from a surface thereof. The second plurality of concentric teeth is spaced apart from the first plurality of concentric teeth by a gap that permits a primary magnetic flux to flow between the first and the second plurality of concentric teeth substantially in a first direction.
The magnetic bearing also comprises a primary magnet magnetically coupled to at least one of the rotor disk and the stator disk and being adapted to provide the primary magnetic flux. The magnetic bearing further comprises a plurality of flux focusing magnets fixedly coupled to at least one of the surface of the rotor disk and the surface of the stator disk and producing a secondary magnetic flux that flows substantially in a second direction substantially opposite the first direction.
Another presently-preferred embodiment of radial magnetic bearing comprises a rotor disk adapted to rotate about a predetermined axis and having a first and a second circumferentially-extending raised portion projecting from a surface thereof, and a stator disk axially spaced from the rotor disk and positioned around the predetermined axis. The stator disk has a third and a fourth circumferentially-extending raised portion projecting from a surface thereof. The radial magnetic bearing also comprises a permanent magnet magnetically coupled to at least one of the rotor disk and the stator disk and providing a primary magnetic flux, a first ring-shaped magnet positioned between the first and the second raised portions, and a second ring-shaped magnet positioned between the third and the fourth raised portions.
Another presently-preferred embodiment of radial magnetic bearing comprises a rotor disk having a first plurality of circumferentially-extending raised portions projecting from a major surface thereof, and a stator disk having a major surface that faces the major surface of the rotor disk. The major surface of the stator disk has a second plurality of circumferentially-extending raised portions projecting therefrom. The radial magnetic bearing also comprises a plurality of flux focusing magnets fixedly coupled to at least one of the major surfaces of the rotor disk and the stator disk.
Another presently-preferred embodiment of radial magnetic bearing comprises a rotor disk adapted to rotate about an axis of rotation and having a first plurality of circumferentially-extending raised portions formed thereon for conducting a primary magnetic flux substantially in a first direction. The radial magnetic bearing also comprises a stator disk positioned around the axis of rotation and axially spaced from the rotor disk. The stator disk has a second plurality of circumferentially-extending raised portions formed thereon for conducting the primary magnetic flux substantially in the first direction.
The radial magnetic bearing further comprises a primary magnet magnetically coupled to at least one of the rotor disk and the stator disk and being adapted to provide the primary magnetic flux. The radial magnetic bearing also comprises a first plurality of flux focusing magnets each being positioned between adjacent ones of the first plurality of raised portions and each being polarized in a direction substantially opposite the first direction, and a second plurality of flux focusing magnets each being positioned between adjacent ones of the second plurality of raised portions and each being polarized in the direction substantially opposite the first direction